Overdrive and Engine RPM
Discussion
Thanks for your reply.
I suspect that a 25% o/d reduces engine speed by 20% for a given road speed.
My guess is that if a car is geared for 20 mph/1000 rpm in direct 4th, at 80 mph its doing 4000 rpm. If you engage a 25% o/d then bring the revs back up to 4000 I think the car would be travelling at 100 mph. If you then bring it down to 80 mph in o/d the revs would be 80% of 4000 which is 3200. The drop from 4000 to 3200 at 80 mph is 20%. If you then disengage the o/d but maintain 80 mph, the engine would be doing 4000 rpm which is an increase of 25%.
The gearing stated in the handbook for my car states that 4th is 1.0 and o/d 4th is 0.82. I've not heard of an 18% overdrive but I think 18% roughtly corresponds to 22% in the same way that 20% equates to 25% in the above example. The Autocar roadtest of the Triumph 2500S also states the gearing of o/d top is 0.82.
None of this is hugely important its just something that's intrigued me for a while.
People I've spoken to so far assume that a 25% o/d reduces engine speed by 25% which I suspect is wrong.
If anyone could clarify I'd be grateful.
Thanks,
Chris
I suspect that a 25% o/d reduces engine speed by 20% for a given road speed.
My guess is that if a car is geared for 20 mph/1000 rpm in direct 4th, at 80 mph its doing 4000 rpm. If you engage a 25% o/d then bring the revs back up to 4000 I think the car would be travelling at 100 mph. If you then bring it down to 80 mph in o/d the revs would be 80% of 4000 which is 3200. The drop from 4000 to 3200 at 80 mph is 20%. If you then disengage the o/d but maintain 80 mph, the engine would be doing 4000 rpm which is an increase of 25%.
The gearing stated in the handbook for my car states that 4th is 1.0 and o/d 4th is 0.82. I've not heard of an 18% overdrive but I think 18% roughtly corresponds to 22% in the same way that 20% equates to 25% in the above example. The Autocar roadtest of the Triumph 2500S also states the gearing of o/d top is 0.82.
None of this is hugely important its just something that's intrigued me for a while.
People I've spoken to so far assume that a 25% o/d reduces engine speed by 25% which I suspect is wrong.
If anyone could clarify I'd be grateful.
Thanks,
Chris
Thanks RCK for lots of great information. I appreciate the time you have taken to put all this together. I now know why a 25% o/d is called 25% and its not all that obvious - not to me at least. If I've understood this it seems a 25% o/d does reduce engine speed by 20% for the same road speed (and increase road speed by 25% for the same engine speed). Its something I've suspected for a while but you have now explained it in terms I can understand. Thank you.
Slightly off topic but I believe there are a couple of TVR 2500s in New Zealand. Its the car I'm trying to understand in a round about sort of way.
Thanks also to //j17. Mintylamb is the sort of site I can lose myself in for hours.
Cheers,
Chris
Slightly off topic but I believe there are a couple of TVR 2500s in New Zealand. Its the car I'm trying to understand in a round about sort of way.
Thanks also to //j17. Mintylamb is the sort of site I can lose myself in for hours.
Cheers,
Chris
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